Method of making a cathode heater assembly



Jan. 4, 1966 C. R. GATEWOOD METHOD OF Filed March 18, 1960 IN VEN TOR. CL YDE H. GA TE W000 A T TORNE Y5 United States Patent ()iiice 3,226,806

Patented Jan. 4, 1966.

2 3 226 806 amethod for embedding heater coils in thermally conductive material having high dielectric strength. I g i gg fi g A still further object of the invention is the provision Clyde Robert Gatewood, San Francisco, Calif., assignor S i f i' eater assembly for t t the cathmie to Eitel McCuliough, Inc., San Carlos, Calif., a corpo- 5 5 i apfifl i the heater C011 benig embedded m ration of California dielectric material in a manner to provide an annular FHedMm; 13 19 9; s 1 02 wafer conforming to the curvature of the dish-shaped Claims. (Cl. 29155.62) Cathode- Another ob ect is the provision of a method of embed Thisinvention relates to cathode-heater assemblies for ding a heating coil having a spheriodal conformation in electron tubes, and particularly to a wire-wound heater 10 a wafer of dielectric material conforming to the curvature coil for indirectly heated cathodes, the heater coil being of the heatingcoil.

embedded in insulating materi l. e invention possesses other ob ects and features of In the electronics industry, it has been a continuing advantage, some of which, with the foregoing, will be set problem to adequately support separate and generally forth in the following description of the invention. It is close-spacedelements which make up an electronic comto be understood that the invention is not limited to the ponent, such as an electron gun Experience has taught embodiment disclosed, as variant embodiments may be t at extremes in temperature, shock, and vibration tend to adopted within the scope of the appended claims. disrupt operation of electronic components due to rupture The problems indicated above have been-obviated by or displacement of such closely spaced elements. Thus, the cathode-heater assembly forming the subject matter in an electron tube using an indirectly heated cathode 0 this invention For purposes of clarity in the descripheated by a cathode heater coil, shocx and vibration may tion, the invention will be described as. embod1ed inv result in rupture of the resistance wire forming the coil, unipotential-type cathode-heater assemblies, particularly thus causing failure of the tube. Instead of being rupadapted for use in the electron guns of beam tubes. It tured the heater coil elements may be displaced, thus will, of. course, be understood that the invention is a bringing them into shorting contact with each other or plicable to other type cathode-heater assemblies. surrounding structure. This is equally detrimental to op- Broadly considered, the cathode-heater assembly formeration of the tube, and it is therefore oneof the objects ing the subject matter of the invention comprises, in the of the invention to provide means for rigidly supporting embodiment chosen for illustration, a dish-shaped catha cathode-heater assembly. 3 ode operatively supported in close proximity to a spirally The prior art discloses that attempts havebeen made to Wound heater. coil. The heater coil is provided with dirigidly support heating elements and cathode heater coils. electric means bonded to the spiral windings and forming y embedding or coating such elements and coils in varielectrical insulation between the cathode and the windous dielectric materials applied by various methods AI"? ings and mechanical support therefor In its preferred most invariably suc coatings and embedding materials form, the spiral windings of the heater coil are axially ave been found to chip and crack forming fissures which displaced in increasing increments to provide the heater often destroy the effectiveness of the coating It is concoil with a spheroidal coufoimation corresponding to the sequently another object of the invention to provide a curvature of the dish-shaped cathode with which it is as wire-wound heater element embedded ininsulating matesociated. The dielectric material, applied in molten form rial, such as alumina in a manner which precludes chipby flame-spraying, immediately hardens and eifectively ping and cracking of the alumina due to differences in retains the heater windings in their spheroidal conforma thermal expansion and contraction of the different matetion Appropriate electrical leads connect the heater coil na to a source of power, and suitable shield means are pro- Ior maximum cathode and heater EfiICICIlCY it is im-v vided to reflect or re-direct heat from the coil back to.- portant that the heater coil follow the contour of the ward the cathode for maximum efliciency. Means arev cat ode. Any material deviation from this conformation also provided cooperating with the shield means to recauses concentrations of heat in different areas of the tain the cathode and-heater in a rigid vibration-free relacathocle, resulting in irregular emission of electrons from. ti-onship possessing maximum heat exchanging efiiciency. the overheated areas This is particularly true as regards Referring to the drawings: cathodes designed for beam tube applications It is th re- F1 is a vertical half-sectional view showing the, fore still another ob ect of the invention to provide a cathode-heater assembly mounte on an appropriate supeater coil shaped to conform to the cathode ith which. port, the support being shown in dash lines. A portion it is associated, and which is embedded in insulating mateof the cathode-heater assembly is broken away to reduce rial which similarly conforms to the shape of the cathode. the sizeof the figure.

It is also desirable that the heater coflhave maximum FIGURE 211 8, a sideelevation 0f the heatercoil .ShOWIl thermal conductivity with relationto the cathode in order as initially wound with the spiral windingslying in planar that a maximum uniform cathode temperature inmini alignment. mum time will be provided. Such thermal conductivity- FIGURE'3 is a plan view of thespirally woundtheate-r is best obtained by placing the heater coil as close as poscoil of FIGUREZ, showing thespacingbetween the indisible and contiguous to the cathode. However, it is also Vidual windings f i he C necessary that the heater coil be electrically insulated in- FIGURE 4 is a transverse sectional view of the heater termediateits ends from the cathode. Thus, to provide coil after the windings have been axially displaced to progood thermal conductivity between heater coil andcath-. vide a spheroidal conformation, and showing the windings )de, it has been found preferable to provide as thin.asecembedded in an annular wafer of dielectric material.

ion of electrical insulation between the heater coil and the FIGURE 5 is a plan view of'the annular wafer shown 'athode as is practicable. It is therefore a still further in FIGURE 4.

bjectof the invention to provide aheater coil embedded FIGURE 1 is shown approximately twice actual size, 1 dielectric material in a manner to provide a surface and FIGURES 2, 3, 4, and.5 are shown approximately. lyer of dielectric material on each side of the heater coil, 1 /2 times actual size.

ie surface layer on the cathode side of the coil being In greater detail and referring to FIGURE 1 of the 'lIlIlfiI' than the surface layer on the other side of the coil rawing, the cathode-heater assembly of my invention 18 having a concave emitting surface 3 and a convex nonemitting surface 4. In the embodiment shown, the cathode is suitably secured to a cathode ring 6 having a radially extending flange 7 and a peripheral cylindrical flange 8. The cathode ring flange 7 is suitably secure-d, as by welding or brazing, to the associated outer peripheral portion of the cathode. Cylindrical flange 8 on the cathode ring is suitably welded or brazed to the inner free end a hollow cylindrical shell 9 forming a support for the cathode and a heat dam to prevent conduction of heat away from the cathode.

The outer end of the heat dam shell is integrally united to the inner end of a hollow cylindrical support shell 12 having apertures 13 circumferentially spaced therearound intermediate its ends to reduce the cross-sectional area of the shell to restrict the flow of heat therethrough. The outer end of shell 12 nests around the cylindrical portion 14 formed on the inner end of hollow conical support shell 16. The union 17 between c lindrical shell 12 and conical shell 16, while shown as being a brazed or welded joint in FIGURE 1, may obviously be a continuous one-piece construction if desired. To rigidly secure the cathode within the tube envelope, radially extending integral base flange 18 on the outer end of the conical shell 16, is suitably brazed to the inner surface of the envelope wall portion 19, shown in dash lines in FIGURE 1. To restrict the conduction of heat away from the cathode, the cylindrical shell 12 and conical shell 16 are preferably fabricated from very thin gauge Nickel and Kovar have been found to possess the requisite thermal characteristics and to retain the desired structura even at the elevated temperatures at which the cathode operates.

To heat the cathode to the desired ture, a heating coil from a length of resistance wire or platinum, conductors are provided joined at the bight, and with terminal leads 23 at their ends lying closely adjacent each other. so that spaced operating tempera- The coil is formed 21, conveniently tungsten The doubled resistance wire is then spirally woun windings in planar alignment are provided. bight 22 is arranged adjacent the outer periphery of the coil, and the coil is spirally wound so that at its two adjacent ends of the resistance wire form the terminal leads 23. Alternately, the bight 22 can be arranged adjacent the inner periphery of the coil and the coil spirally wound outwardly in order to place the terminal leads 23 near the outer periphery of the coil.

In order to displace the windings to provide a spheroidal conformation corresponding to the convex curvature of the cathode 2, the coil is mounted in a suitable jig (not shown) and the windings are axially displaced varying amounts or in increasing increments commencing from the outer periphery of the coil and ending at the inner periphery. While retained in this position on the jig, the coil is flame-sprayed with molten ceramic material, preferably high dielectric strength alumina, the dielectric material hardening into an annular electrically insulating and mechanically rigid supporting wafer 24.

their proper relationship while they are being sprayed with dielectric material, a platen (not shown) similar in to the curvature of the cathode surface with which the coil will ultimately be associated, is pressed downwardly on the coil and depresses the separate windings of the coil the proper amount to provide the desired spheriodal conformation. is then flame-sprayed with ceramic material to retain the coil windings in position. Alternately, the windings while in planar alignment, as shown be given a light coating of flame-sprayed ceramic mate nd the light coating permitted to harden. The lightly coated coil is then pressed into the desired spheroidal conformation as previously explained. At this point each of the windings will be parallel found to be surrounded by a thin layer of hardened ceramic materia in the order of about .010 inch thick. It has been found that displacing the windings to form a spheroidal conformation does not crack nor chip the thin coating of ceramic material. Once having displaced the lightly coated windings to the desired curvature, the coil is again flame-sprayed with molten ceramic material from the side opposite the platen which holds the coil in position. This results in a thicker coating of ceramic material being built up on the back or convex side of the coil, while a relatively thinner layer of ceramic material covers the coil windings on of the coil. This condition is best shown in FIGURE 1. When the spaced windings are sprayed with molten dielectric material, the dielectric materlal builds up on the surface of the wire, forming a depression in the space between the windings. If desired, spraying may be continued until these depressions are substantially filled on the back or convex side of the coil; however, on the concave side it is desirable that the depressions be permitted to remain. It has been found that on the concave side of the heater coil it is desirable that the ceramic coating interposed between the coil windings and the underside of the cathode be as thin as possible. This places the heater windings as close to the convex surface 4 of the cathode as is practicable, there being only a .010 inch thickness of dielectric material separating the windings from the cathode. It will thus be seen that the layer of dielectric material functions in the manner of a manifold to evenly distribute heat from the heater windings and efliciently conduct it to the cathode by reason of its contiguous relation thereto. Radiation of heat from the depressions aids in providing a uniform temperature throughout the cathode. Forming the ceramic Wafer in this manner precludes cracking or chipping due to differences in expansion and contraction between the ceramic and the heater coils. This result has not been achieved where the ceramic in a plastic state is cold-sprayed or cast around the heater windings and then fired to harden. In the latter case it has been found that when the heater windings are subsequently heated, the differences in expansion and contraction result in cracking and destruction of the ceramic wafer. The reason why the ceramic coating of my invention is not affected in this way is believed to be due to the phenomenon that when the molten ceramic is flamesprayed onto the heater windings, the heat from the molten ceramic causes the wire to expand. The ceramic material is also expanded because of its molten condition, but upon cooling after being sprayed, it cools faster than the wire which it surrounds. The ceramic having cooled and hardened and therefore being possessed of some dimensional stability, the wire contained within the ceramic then slowly cools and is believed to contract and shrink, stressing the bond between the embedded resistance wire and the surrounding dielectric material. In some instances the resistance wire may contract sufficiently to form a void between the wire and the surrounding dielectric material. When the heating element is subsequently heated by passing an electric current therethrough, the thermal expansion of the wire merely relieves the stress on the bond or fills the void if such there be. When maximum expansion of the wire has been achieved, it is believed the tensile stress imposed on the ceramic wafer is well below the destructive value that would normally be imposed if the stress in the bond did not have to be overcome or if a slight void did not exist. It will thus be seen that the ceramic material is stressed much less than it would be if the ceramic in a plastic state were cold-sprayed or cast about the cold heater windings. Means are provided to secure the embedded heater coi' in operative position beneath the convex surface of tht cathode. Fitting snugly over each terminal lead 23 is 1 ceramic bushing 26 having a flange 27 at its upper en abutting the under surface 28 of the dielectric wafe adjacent its inner periphery. Fitting snugly about th main body portions 26 of the bushings are a plurality of sulate and mechanically support said coil windings in dish-shaped heat shields 29 substantially complementary said spheroidal conformation.

to the shape of the cathode. Each of the heat shields 1's 3. The method according to claim 2, in which said provided with an aperture through which the body portion dielectric material comprises alumina, and said windings 26 of the bushing extends, and each is additionally pro- 5 are embedded by spraying molten alumina thereon.

one above the other in operating position under the dielectric material in molten form on resistance wire, cathode. To support the heater coil and heat shields in allowing said coating to harden, forming said wire into a operating position a bottom support shield 32 is provided. desired shape having spaced adjacent turns, and then the parts against movement due to vibration and shock, to the wire from the side of the wire opposite said brazed in the upper end of a conductive sleeve 36 which shape having spaced adjacent turns, and said thin coatcontinues downwardly and extends out of the envelope mg being applied in lHSllfiiClel'lt quantity to inte connect through the support plate 19, to be connected to a source said turns. of power to pass aheatingcurrent through the coil. To 7. The method according to claim 6 in which said complete a conductive circuit through the heating coil, desired shape is obtained by pressing the wire against the other terminal 23 is welded to one end of strip cona spheroidal platen, and said second coating is applied ductor 37, which at its other end is welded to the flange to form a greater thickness of dielectric material on the 33 on the support shield 32. Support shell 12 and conical convex side of the heater.

shell 16 complete the circuit.

From the foregoing it will be obvious that an efficient References Cited y the am er iugd riid dcat1hodeheaterd assemal; has been fpgogided. UNITED STATES PATENTS e m 1V1 ua pars are esigne or aciiy o a IlCation, and after fabrication the parts cooperate to provide g 5 a vibration-free structure. Such vibration-free construc- 2504335 4/1950 Junker 340 tion ensures long and efiicient life for the cathode-heater 2'677873 5/1954 B k 2 bly 40 uc 9 25.17

iig g 2,724,788 11/1955 Edwards 313-34o X 1 The method of fabricating a heater coil for electron s gi et a]. 51?? tubes comprising winding a length of resistance wire to 2760077 8/1956 Lo g 7 T 66 provide spaced windings in planar alignment, displacing 1 1 82 7/1957 0 selected windings in varying increments to provide said Os 40 coil with a spheroidal con-formation, and depositing 2,888,591 5/1959 schmldt F 3 313-337 molten dielectric material on said windings while so dis- 2,966,430 12/1960 scbrewehus 117-215 placed to form a dielectric coating thereon to electrically 3,137,766 6/1964 Teague 117-231 insulate and mechanically support said windings. FOREIGN PATENTS 2. The method of fabr1cating a heater coil for electron 1:012796 7/1957 Germany.

wire to provide a multiplicity of radially spaced spiral 179216 4/1922 Great Britain windings i P axially S F t JOSEPH B. SPENCER, Primary Examiner.

spheroidal conformation, and embedding said windings in RALPH G. NILSON, RICH ARD D. NEVIUS, WIL- a dielectric material while so displaced to electrically in- LIAM D. MARTIN, Examiners. 

1. THE METHOD OF FABRICATING A HEATER COIL FOR ELECTRON TUBES COMPRISING WINDING A LENGTH OF RESISTANCE WIRE TO PROVIDE SPACED WINDINGS IN PLANAR ALIGNMENT, DISPLACING SELECTED WINDINGS IN VARYING INCREMENTS TO PROVIDE SAID COIL WITH A SPHEROIDAL CONFORMATION, AND DEPOSITING 